As a main carrier and a material basis for information exchange and transmission, display devices, such as liquid crystal display (LCD) device and organic light emitting diode (OLED) display device, draw attention of more and more people, and are widely used in various aspects of work and life. In recent years, flexible display becomes popular among consumers in the industry, and in particular, industries of flexible thin film transistor liquid crystal display (TFT-LCD) and flexible organic light emitting diode (OLED) have gradually become most promising high-tech industries. Various kinds of flexible display devices, such as television screen, mobile phone screen, wearable device screen, large-sized display screen for business presentations, computer screen and so on, are proposed by different research institutes and enterprises. Liquid crystal display device and organic light emitting diode display device, as two important development fields of the flexible display device, face same problems during development process, i.e., a selection of materials of a flexible substrate and manufacturing thereof.
In order to achieve a flexible display technique, it is required to not only improve design and manufacture techniques but also enhance research, development, and industrialization of various critical materials. The flexible substrate, as a component to support and protect a flexible device, not only has an important influence on display quality of the flexible device but also directly relates to lifetime of the flexible device. Thus, research and development of the flexible substrate is crucial for development of flexible display.
The flexible display device requires a substrate material to have the following properties. (1) The substrate material should have heat resistance and dimensional stability under high temperature, which is determined by a manufacturing procedure of a display screen. High temperature in the manufacturing procedure may cause hot deformation of a substrate, which reduces precision of positioning. Moreover, an excessive variation of a dimension of the substrate may cause unsatisfactory display effect due to an internal stress between surfaces of materials and a separation between layers when the substrate is bent. (2) The substrate material should have flexibility. The development trend of the flexible display device is from a flat type to a bendable type, to a rollable type, and then to a foldable type. A bending radius of the substrate will become increasingly small, and the substrate will become increasingly thin. (3) The substrate material should have a water-proof and oxygen-proof ability. When exposed in an environment of water vapor and oxygen, performance of a display medium of the flexible display device, organic materials in the organic light emitting diode in particular, may degrade rapidly. Thus, water vapor permeability and oxygen permeability of the flexible substrate should be as low as possible. (4) The substrate material should have a flat surface. The surface quality of the substrate, such as roughness degree and clearness degree of the substrate, has a great influence on the mechanical performance of the substrate and the display effect of the display device.
At present, substrate materials of the flexible display device mainly include ultra-thin glass, metallic foil, polymeric film and so on. Considering requirements for the properties of the flexible substrate on the whole, the polymeric film has distinct advantages in optical performance, mechanical performance, chemical performance and so on. Moreover, mass production of the polymeric film can be achieved. Besides, the polymeric film is applicable for a substrate with any size, and the cost thereof is relatively low. Thus, the polymeric film draws wide attention in development of the flexible display device. Commonly used polymer substrate materials include polyethylene glycol terephthalate (PET), polycarbonate (PC), polyimide (PI) and so on. The polyimide substrate material draws great attention for its favorable heat resistance, permeability, mechanical performance, and chemical stability. Rigid polyimide ring of polyimide provides polyimide excellent comprehensive performance, and thus polyimide becomes a first choice of material for the flexible substrate.
However, a traditional polyimide film has great phase retardation. For example, the polyimide film formed by spin coating method has phase retardation in thickness direction, and with increasing of the thickness, the phase retardation increases. That is, the polyimide film has birefringence effect. For the display device, the phase retardation (or birefringence) of a flexible substrate in the thickness direction may easily cause light leakage from a side view direction, which affects the display effect.